This invention relates to dynamoelectric machines such as large AC induction or synchronous motors with multiple turn coils. The invention may also be applied to other machines, such as turbine generators and waterwheel generators, with multi-turn windings.
In dynamoelectric machines, coils are placed in slots in a magnetic core. The coils are made up of a number of turns and the coil turns in a slot are insulated from each other and from the adjacent core. In a large machine, such as a motor of at least several thousand horsepower rating, the coil turns with their individual turn insulation are usually stacked one on top of another in a coil slot and the entire number of coil turns in the slot for a particular coil are further covered with ground wall insulation for insurance against a conductive path occurring between any of the turns and the adjacent core.
An insulation system for such motors may use various types of insulating materials. One example for high voltage applications is epoxy resin impregnated mica paper used for both the turn insulation and for the ground wall insulation. Machines having such insulation systems have long been successfully used. There are, however, some occurrences of failure of such machines due to abnormal transient events on the power system with which the machine and its coils are connected. This may happen when a voltage is abruptly applied to the machine. This condition is referred to as a steep front impulse, such as one having a rise time to the full magnitude of the applied voltage of about 0.1 to 0.3 microsecond. In such an instance, there is a brief time in which different turns of a single coil may be at significantly different electrical potential. If the voltage difference is great enough, there can be a breakdown or discharge between the involved conductors leading to destruction of the insulation. The voltage can reach about 40 KV or higher where the normal line voltage is about 14 KV and surges are anticipated to reach some multiple of that voltage.
The present invention has to do with a machine having coil turns arranged in combination with an insulation system that selectively increases the turn-to-turn insulation strength. That is, by this invention, it is not merely the case that the insulation strength is increased by greater thickness of insulating material or improved insulating material on each of the conductors but rather by interposing additional insulating material between conductors that are subject to the breakdown phenomenon. The selectivity in applying the additional insulation is important so it requires essentially no redesign of motors as far as conductor size or slot size is concerned. In one aspect, therefore, the invention has to do with the provision in an existing coil slot design of conductors with an insulation system that substantially prevents the occurrence of breakdown between coil turns that may be of widely different voltages due to a steep front impulse voltage.
Tests of coils have been conducted that have been constructed in accordance with the above description of prior art in which there is individual turn insulation and ground wall insulation. Steep front impulse voltages were applied with increasing voltage magnitude in successive applications until failure of the insulation. The coils were dissected and the location of the breakdown determined. In machines of a design that has been widely used in large motors, such as some of 11,000 horsepower, which include coil turns numbering about six in a coil slot in a linear stack, it was found that the coils failed ultimately turn-to-turn, that is, there was no instance of failure between a coil turn and the core. However, the failures did not occur between adjacent turns since the voltage between adjacent turns apparently is not high enough to puncture the turn insulation. Instead, the turn-to-turn failure was between the highest voltage turn, such as the first turn of the stack, and the lowest voltage turn, such as the last turn of the stack or the ground turn. What appeared from examination of the coils was that the discharges propagated from the first conductor through its turn insulation and along a path between the turn insulation of the subsequent turns and the ground wall insulation until reaching the ground turn whose turn insulation was also punctured. As the discharge propagates towards the ground turn, the voltage across the turn insulation increases in the absence of any appreciable voltage drop in the discharge.
An insulation material such as epoxy resin impregnated mica paper can itself have a very high insulation strength making a discharge difficult to propagate through it. However, in systems as described in which a ground wall insulation of such material is wrapped and processed around a stack of coil turns that each have individual turn insulation, it appears likely from the tests conducted that the minor voids and cavities between the ground wall insulation and the turn insulation facilitate the propagation of the discharge. Such voids and cavities would not normally be considered to make the insulation system defective and their elimination in the manufacturing process would be a sustantial expense.
The present invention will substantially eliminate the type of failures referred to. Briefly, in accordance with the present invention, in a given coil slot and conductor configuration, the insulation system is modified to include three significant parts rather than the two part system as was previously used on these machines. In addition to the turn insulation and the ground wall insulation, an additional layer of insulation encompasses groups of coils of the coil stack. For example, the coil turns may be simply divided into two groups, each of roughly half of the coil turns, and that group of turns with its individually insulated conductors is covered by its own layer of insulation, here referred to as group insulation, so that each of the turns of a group are further insulated from each of the turns of the other group by the material and thickness of the group insulation.
It is significant that the addition to the insulation system of the group insulation is provided by the present invention without requiring wider dimensioned coil slots. This result occurs because the final ground wall insulation is simply made thinner in order to accommodate the group insulation. This still provides, assuming all the materials are of the same quality in each part of the insulation system, the same amount of insulation between each coil turn and the core. A minor addition to the dimensional height of the coils and their insulation occurs as a result of the two additional layers, one on each group, of group insulation that is interposed between the two groups of coil turns. However, this variation in height as opposed to prior machines can be readily accommodated by adjustment of the dimensions of spacers or wedges that are employed at the slot opening to tightly close the slot and fill the space therein.
Wieseman U.S. Pat. No. 2,201,845, May 21, 1940, is of some interest as background to the present invention. It is directed to coil conductors with turn insulation and ground wall insulation plus additional layers of insulation (e.g. 27, 29, 31 in FIG. 4 of said patent) interposed respectively between adjacent turns, with increasing thickness of the additional layers. However, conventional manufacturing processes will, in such a structure, leave minor voids and cavities between the turn insulation and the ground wall insulation and also between the edges of the additional layers of insulation and the ground wall insulation. Consequently, the arrangement of the patent is not regarded as effective in avoiding steep front impulse breakdown. In contrast, by the present invention, any discharge would have to penetrate through both the turn insulation and the group insulation. If it merely penetrates through the turn insulation, there is not a path along which it could propagate to the turn insulation of another conductor.
That is, while the arrangement of the patent alters the capacitance between turns and provides an increased creepage path along the interface between the turn insulation and ground insulation, it does not provide a positive dielectric barrier to voltage as does applicants' group insulation.